This invention relates generally to a system of sewage treatment and transport in which the sewage undergoes initial treatment at or near its point of generation to condition it for transport through a collection system to a central treatment plant where, after the completion of treatment, the innocuous liquid remaining is discharged into a stream, lagoon, or other body of water, or may be utilized for irrigation or such other purposes where the use of recycled water is permitted.
The term "sewage" as used herein is defined as the liquid waste containing both dissolved and suspended solids resulting from the discharge of toilets, baths, sinks, laundry tubs, and other fixtures in residential building or commercial establishments. Although the quantities of dissolved and suspended solids are relatively small, they contain substantial amounts of organic material which are putrescible and which may give off foul and corrosive gases if not treated promptly. For this reason, sanitary sewerage systems have customarily been designed to provide a gravity flow of the sewage from the point of entering the system to its final discharge.
Such systems are normally designed to provide velocities of at least 2 feet per second to ensure the prompt arrival of the sewage at the treatment plant or disposal site. Large amounts of water are also required to carry the solids at the velocity through the gravity system.
In hilly terrain sufficient natural differences in elevation normally exist to effectively permit the gravity flow of sewage. However, where sufficient natural differences in elevation do not exist, sewage is collected in sumps or wet wells at pump stations at one or more low points in the system, from which it must be pumped through force mains toward the treatment plant or outfall point.
Gravity sewers are constructed of relatively large diameter pipes so as to accommodate peak flows and so as to avoid being obstructed by the passage of solids contained in the sewage which are frequently stranded in the pipe system during periods of low flow, and are subsequently recaptured during later periods of high flow.
Where sewers are constructed to serve a sparsely settled area or one where there is little natural slope to permit adequate gravity flow, sewage remains for long periods in the collection system with the result that it becomes septic and solids accumulate to cause stoppages within the system. Under such conditions the operation of the system becomes difficult and expensive as foul and corrosive gases cause severe corrosion within the collection system and odor nuisances at and in the vicinity of pumping stations or treatment plants, and severe objections by residents in the neighborhood, which with the forthcoming programs for water and energy conservation will become more severe.
An inherent fault with the gravity collection of sewage is the leakage of water from outside of the sewer pipe into the system through the numerous joints between the individual pipes and fittings. Such infiltration will vary with the type of sewer construction and the relative location of the sewer to the groundwater table. In extreme cases infiltration can severely restrict the capacity of the sewer for receiving sewage. Although moderate quantities of infiltration will improve the flow in an underutilized sewerage system, capacity for its treatment must be provided at the treatment plant with the corresponding increase in the cost of system operation. Cost of treatment of groundwater infiltration often reaches fifty percent of the entire plant operation.
The requirement that a gravity sewer system maintain a continually downward gradient throughout its length can result in high system costs as deeper trenches and hard and expensive excavation is encountered in the lower reaches of a system. This may become even more critical with water saving devices at the home.
The typical gravity sewer must be constructed initially providing for its ultimate capacity so that a heavy financial burden is placed on a growing community in the early years of its existence, when funds for the payment of capital expenses are difficult to obtain and often limit the ability of a community to provide such a needed service. Furthermore, the construction of the ultimate required capacity results in the underutilization of the system and causes an undue financial burden on everyone involved. This is especially true now that government grants for sewer construction are being limited to present needs and are based on a coast effective analysis.
Although the sewage treatment plant makes up a substantial part of the overall cost of providing a complete sewerage system, it has generally been possible to construct it in stages paralleling the growth of the area served by the system so as not to be an undue financial burden.
Current trends toward seeking a cleaner environment have resulted in the need for more extensive and sophisticated methods of waste water treatment.
Methods developed to meet these requirements are progressively more expensive to construct and operate, require greater technology and are more labor intensive, with the result that the cost of sewage treatment assumes a far greater financial burden on the system users as the demands for improved treatment continue.
The increase in cost to provide advanced wastewater treatment and the need to provide for large diameter pipes in the conventional collection system quickly place the cost of providing proper sewerage facilities beyond the economic means of small and growing communities.
Since public health considerations require that all citizens be provided with a safe water supply free from enteric organisms, the extension of sewerage systems to serve all dwellings in small and growing communities becomes imperative and means to accomplish it must be provided.